(1) Field of the Invention
The invention relates to a method for optimizing the point at which an air conditioning unit is switched between operating in a chiller mode to operating in a free cooling mode.
(2) Description of the Related Art
During the typical operation of air conditioning units, the air conditioning unit is run in a chiller mode wherein energy is expended to chill the air within a volume to be cooled. However, when the ambient temperature is low, there exists the possibility that such a low temperature ambient air may be utilized to provide cooling without engaging in a chiller mode. When low temperature ambient air is used by an air conditioning unit to cool a space, the unit is referred to as operating in a free cooling mode. As noted above, traditionally, even when ambient air temperature is low, the air conditioning unit is run in a chiller mode. Running in chiller mode under such conditions provides a low efficiency means of air cooling. In contrast, running the air conditioning unit under such conditions in a free cooling mode is more efficient. In the free cooling mode, one or more ventilated heat exchangers are activated and the water circulating throughout the air conditioning unit is cooled by ambient air.
Unfortunately, it is difficult to estimate the optimum outdoor temperature at which free cooling may be most effectively and efficiently utilized. This optimum point corresponds to the moment when the building load is equal to or lower than the maximum capacity of free cooler. Building load refers to the amount of energy required to be removed to keep the space, for example the interior of a building, at a constant or desired temperature. Free cooler maximum available capacity refers to the amount of energy capable of being removed by the ambient cool air. One problem arising when trying to determine the optimum outdoor temperature at which an air conditioning unit should switch from chiller mode to free cooling mode arises from the fact that free cooler capacity is directly related to outdoor air temperature while a building load may not be. Specifically, for the same outdoor temperature, the occupation of a building, as well as the heat generated internally, may significantly vary.
When free cooling is an option, it is preferable to operate in a free cooling mode, as opposed to a chiller mode, when operating a chilled water air conditioning system. Operation in a free cooling mode helps to achieve significant energy savings. In many applications, cooling is required even if the outdoor temperature is low. Operation in a free cooler mode consists of utilization of “dry cooler” (ventilated water to air heat exchanger) on the chilled water loop instead of utilizing a chiller. This arises from the fact that when outdoor temperature is lower than the water loop temperature of the system, it is possible to transfer heat from the water loop to ambient air using forced convection without the utilization of a chiller.
In traditional applications, there is set an arbitrary ambient temperature level at which a switch is performed between chiller mode and free cooler mode. When the ambient temperature reaches this arbitrarily set level, the chiller is turned off and free cooling mode is initiated. If this arbitrary temperature is not in phase with the actual needs of cooling the building, there can result an unstable operation. Specifically, there may be encountered a situation wherein there occurs rapid switching back and forth between free cooling mode and chiller mode. In addition, it may arise that cooling is performed in a non-optimum manner such as when the air conditioning unit is run in chiller mode when free cooler mode is both available and preferable.
What is therefore needed is a method for determining a point at which it is efficient to switch from chiller mode to free cooling mode in an air conditioning unit.